Abstract view
Convex Polynomial Approximation in the Uniform Norm: Conclusion


Published:20051201
Printed: Dec 2005
K. A. Kopotun
D. Leviatan
I. A. Shevchuk
Abstract
Estimating the degree of approximation in the uniform norm, of a
convex function on a finite interval, by convex algebraic
polynomials, has received wide attention over the last twenty
years. However, while much progress has been made especially in
recent years by, among others, the authors of this article,
separately and jointly, there have been left some interesting open
questions. In this paper we give final answers to all those open
problems. We are able to say, for each $r$th differentiable convex
function, whether or not its degree of convex polynomial
approximation in the uniform norm may be estimated by a
Jacksontype estimate involving the weighted DitzianTotik $k$th
modulus of smoothness, and how the constants in this estimate
behave. It turns out that for some pairs $(k,r)$ we have such
estimate with constants depending only on these parameters. For
other pairs the estimate is valid, but only with constants that
depend on the function being approximated, while there are pairs
for which the Jacksontype estimate is, in general, invalid.